1. Field
The present invention relates generally to data communication, and more specifically to techniques for protecting against merging of demodulation elements (or fingers) assigned to process multipath components of a received signal in a wireless communication system.
2. Background
In a wireless communication system, a modulated signal transmitted from a transmitter may reach a receiver via a number of signal paths due to artifacts in the propagation environment (e.g., building, natural structures, and so on). The received signal at the receiver may thus include a number of multipath components, each of which corresponds to a signal instance received via a particular signal path. Since the receiver may also receive signals from multiple transmitters, the received signal may include a number of multipath components for a number of transmitters.
For a CDMA system, a rake receiver is often used to concurrently process a number of multipath components in the received signal. The rake receiver normally includes one or more searcher elements (or simply “searchers”) and a number of demodulation elements (often referred to as “fingers”). Each searcher may be operated to process the received signal to search for strong multipath components. Each finger may then be assigned to process a particular multipath component of sufficient strength. By assigning multiple fingers to process a number of multipath components and then combining symbol estimates from all assigned fingers, more of the energy in the received signal may be collected and improved performance may be achieved.
Each multipath component is associated with a particular propagation delay and a particular time of arrival at the receiver, both of which are determined by the signal path. For a wireless communication channel, and especially for a mobile environment, the signal paths may continually change over time. Thus, as part of the signal processing, a time tracking loop is normally maintained for each active finger and used to track the timing of the multipath component assigned to the finger. Each time tracking loop would advance or retard its timing, as necessary, so that it remains locked to the changing arrival time of the assigned multipath component.
As the signal paths change, the arrival times of two or more multipath components may approach close to each other. When the difference between the arrival times of the multipath components is equal to or less than a particular threshold, the multipath components appear as an energy “blob” and the assigned fingers are not able to resolve them. Consequently, these fingers would end up tracking the same multipath component, a phenomenon often referred to as “finger merging”. This merging effect is undesirable because the fingers would not be able to track the individual multipath components when the signal paths subsequently change and the multipath components move away from each other. Once merged, the fingers cannot be easily separated, and the benefits obtained by assigning these multiple fingers to process the received signal are lost.
There is therefore a need in the art for techniques to prevent the fingers of a rake receiver from merging together.